This invention relates generally to liquid pumps and apparatus for permitting the addition of a secondary liquid to the primary pump liquid at the pump inlet. The invention relates specifically to positive displacement pumps such as piston or diaphragm pumps wherein a reciprocating member creates variable liquid pressure impulses and thereby accepts primary liquid at its inlet under varying flow rate conditions.
U.S. Pat. No. 2,389,134, issued Nov. 20, 1945, discloses a simple fluid flow control device by means of which a constant rate of fluid flow through the device is secured, irrespective of the pressure variation of the fluid delivered to the flow control device. The device comprises a flexible member having an opening therethrough which is smaller in cross-sectional area than a fluid flow passage into which the device is inserted. The device typically rests upon an internal shoulder in the fluid flow passage, which shoulder provides a partial supporting structure for the flexible member, although the opening through the member is always smaller than the opening created by the projecting shoulder. When fluid is passed through the passage in which this device is supported the pressure of the fluid upon the upstream face of the flexible member causes the member to become distorted in a downstream direction, thereby reducing the cross-sectional area of the member opening exposed to the fluid flow. This reduction in opening causes a corresponding increase in pressure drop to ensure a relatively constant liquid flow rate through the device, and the device operates on the principal that its interior opening is inversely proportional to the liquid pressure sensed by the device to thereby maintain a constant liquid flow rate therethrough.
U.S. Pat. No. 2,454,929, issued Nov. 30, 1948, disclosed an improvement over the foregoing device by constructing the passage immediately downstream of the flexible member into a conically shaped seat to permit greater deflection of the member for improved liquid flow control. U.S. Pat. No. 2,899,979, issued Aug. 18, 1959, disclosed a further improvement over the foregoing invention by incorporating a supporting metal ring into the flexible member to control the flexible member distortion and thereby enable the use of the device over higher liquid pressure and volume ranges.
Other inventions have further modified and improved the basic flow control device, but in its essential attributes it provides liquid flow rate control utilizing the principal of a flexible member having a deformable passage of cross-sectional area which is inversely proportional to the liquid pressure on the member. In the present invention this principle is combined with a secondary liquid feedpipe placed downstream of the flexible device, both of which are placed at or near the inlet port of a reciprocating pump. The action of the combined members in response to the normal pressure fluctuations caused by the reciprocating pump creates a region of low or negative pressure downstream of the flexible member for drawing secondary fluid flow into the primary fluid flow passage.
It is well known that a positive displacement pump of the reciprocating type, particularly a diaphragm or piston pump, has an undulating flow rate both at the intake port and the discharge port. This undulating flow rate is caused by the generally sinusoidal flow characteristics of the pump, wherein flow starts at zero at the pump dwell point, rising to maximum flow at the midpoint of the intake stroke and falling back to zero at the second pump dwell point. It is generally understood that the peak flow rate during the pump intake cycle and during the pump discharge cycle is considerably greater than the average flow rate during the complete cycle. In other words, a pump which is rated at two gallons per minute discharge flow rate will intake liquid at a peak rate of about three gallons per minute during the intake stroke and similarly discharge at a peak rate of about three gallons per minute on the discharge stroke. This undulating flow is generally smoothed out by some type of surge suppressor on the inlet and outlet side of a reciprocating pump. However, a surge suppressor typically only reduces, without eliminating, fluctuations in liquid flow rate during pump operating.
The present invention provides a simple and inexpensive apparatus for introducing a liquid additive into a primary flow liquid for pumping and mixing. For example, it provides an apparatus for introducing liquid soap into a water passage wherein the mixture is drawn into a reciprocating pump and pressurized for cleaning purposes. In the prior art liquid additives which were handled in this manner required a pressure regulator on the pump intake line to control the inlet pressure, or a line restriction of some sort to control the total flow through the pump inlet. This has the disadvantage of being extremely difficult to balance to provide the proper pressure without introducing pump cavitation. The present invention overcomes this disadvantage by causing a negative pressure during only a small portion of the pump intake stroke, which negative pressure may be controlled as to time and magnitude so as to control the amount of additive introduced as well as to permit enough positive pressure on the remaining pump intake stroke to prevent cavitation.
This invention relates generally to positive displacement pumps which develop periodic flow surges through their normal operating cycles. It is particularly adapted for use with a diaphragm pump of the type generally disclosed in U.S. application Ser. No. 701,807, filed July 7, 1976 and owned by the same assignee as the present invention.